What does a dip at 4000 Hz on an audiogram indicate in a patient with otherwise normal hearing acuity and a history of sensorineural hearing loss?

Normal Hearing with a 4000 Hz Dip: Clinical Significance and Implications

A dip at 4000 Hz on an audiogram with otherwise normal hearing thresholds is the characteristic signature of early noise-induced hearing loss (NIHL), representing irreversible damage to outer hair cells in the organ of Corti, though this pattern can also occur from other causes including acoustic trauma, ototoxicity, or even without identifiable etiology. 1, 2

Understanding the 4000 Hz Notch Pattern

The 4000 Hz audiometric notch (also called "c5-dip") represents a specific configuration where:

• Hearing thresholds are ≤20 dB HL at most frequencies (normal range) 3
• A distinct dip or notch occurs specifically at 4000 Hz, typically ≥10 dB worse than adjacent frequencies at 2000 Hz and 8000 Hz 4
• This pattern reflects damage to the basal turn of the cochlea where high-frequency sound processing occurs 1, 2

The notch at 4000 Hz is considered pathognomonic for noise exposure because this frequency corresponds to the region of maximum cochlear damage from acoustic energy, though the evidence shows this pattern is not exclusively caused by noise. 5, 4

Clinical Significance and Etiology

Primary Cause: Noise-Induced Hearing Loss

The 4000 Hz notch is the earliest detectable sign of NIHL, appearing before hearing loss becomes symptomatic or affects speech frequencies. 1, 2

• Occupational noise exposure exceeding 85 dB(A) is the most common cause 1, 2
• Recreational noise exposure from nightclubs, concerts, or personal audio devices at 92-95 dB(A) produces similar damage 1
• The notch represents permanent threshold shift (PTS) indicating irreversible outer hair cell damage 1

Alternative Etiologies to Consider

The evidence reveals important nuances that challenge the assumption that all 4000 Hz notches are noise-induced:

• Acoustic trauma from single loud events can produce identical patterns 5
• Ototoxic medications (cisplatin, aminoglycosides) characteristically affect high frequencies including 4000 Hz 1
• Unknown etiology accounts for a substantial proportion of cases—in one study, 62 of 159 patients (39%) with 4000 Hz notches had no identifiable cause 5
• The configuration of the notch (cone, bowl, or dish type) appears similar regardless of underlying cause 5

Prevalence and Laterality Patterns

Unilateral 4000 Hz notches are nearly twice as common as bilateral notches, occurring in approximately 40% of adults with hearing evaluations. 4

• In a veteran population study of 3,430 individuals: 15.4% had bilateral notches, 28.8% had left ear notches only, and 27.1% had right ear notches only 4
• The prevalence peaks in the 40-50 year age groups (approximately 35% of individuals) and diminishes in older age groups 4
• Mean notch depth consistently measures 20-26 dB across age groups 4

This high prevalence of unilateral notches challenges the traditional noise-exposure explanation, as occupational noise typically affects both ears symmetrically. 4

Critical Diagnostic Considerations

Confirm Sensorineural Pattern

Bone conduction testing must be performed to verify this is sensorineural hearing loss and not conductive pathology. 2, 3

• Cerumen impaction must be excluded through otoscopy before establishing any diagnosis 6
• Tympanometry should confirm normal middle ear function 1, 3

Assess for Measurement Artifact

The 4000 Hz notch can be artifactual when using supra-aural (TDH-50P) headphones, particularly in individuals with shorter external auditory canals. 7

• Real-ear sound pressure level (RESPL) calibration errors at 6000 and 8000 Hz, influenced by shorter ear canal length, are major determinants of artifactual notches 7
• Insert earphones (ER-3A) produce substantially lower prevalence of notched audiograms compared to supra-aural receivers 7
• Standard test-retest variability fluctuates by ±5 dB, so changes <10 dB may not represent true threshold shifts 3, 6

Document Noise Exposure History

Thoroughly document occupational and recreational noise exposure, including duration, intensity, and hearing protection use, as this is essential for diagnosis and potential medicolegal implications. 2

• Daily noise exposure (LEX,8h) in entertainment venues reaches 92-95 dB(A), approximately 4 times higher than legally accepted limits 1
• Workers exposed to noise exceeding 85 dB(A) require mandatory hearing protection 2

Management and Prevention Strategy

Immediate Interventions

Remove the patient from noise exposure exceeding 85 dB(A) immediately or mandate consistent use of hearing protection devices (earplugs and/or earmuffs). 2

• For patients with documented hearing loss, implement hearing protection at 80 dB(A) or above rather than the standard 85 dB(A) threshold, as existing damage increases susceptibility 2
• Engineering and administrative controls should be the first line of defense before relying solely on personal protective equipment 2

Surveillance and Monitoring

Institute annual audiometric testing at 3000,4000, and 6000 Hz frequencies to monitor for progression. 1, 2

• Monitor for temporary threshold shifts (TTS) after work shifts to detect ongoing damage before permanent hearing loss worsens 1, 2
• The hearing loss already present is irreversible; the goal is preventing further deterioration 2

Screen for Synergistic Risk Factors

Assess for co-exposure to ototoxic substances (styrene, toluene) which synergistically worsen hearing loss when combined with noise. 2

• Screen for hypertension, diabetes, and elevated lipids, as these conditions increase individual susceptibility to noise-induced hearing loss 1, 2
• Document cigarette smoking status, which independently increases risk of noise-induced progression 2

Common Pitfalls to Avoid

• Do not assume all 4000 Hz notches are noise-induced without considering ototoxicity, acoustic trauma, or idiopathic causes 5
• Do not overlook unilateral notches as insignificant—they are more common than bilateral patterns and warrant full evaluation 4
• Do not use supra-aural headphones for screening young adults when investigating NIHL prevalence, as calibration errors produce false-positive notches 7
• Do not fail to counsel patients that existing hearing loss is permanent and prevention of further damage is the only therapeutic option 2